Physiological data can be measured from a user by using portable biometric monitors, which may be attached to the user, for example to the chest, wrist, forearm, or arm of the user. The physiological data may include for example heart rate. Traditional monitors usually contain a separate sensor, which is attached for example to the user's chest with a strap, and which communicates wirelessly with a wrist device. The use of separate sensors complicates the use of a portable biometric monitor, and therefore there is a need to develop solutions embedded to the wrist-attached or other extremity attached device.
One way for measuring the heart rate is using optical measurement. The optical heart rate measurement is based on the fact that light is emitted by a light source towards body tissue and at least one detector is configured to detect the intensity of reflected light after propagation through the body tissue.
In such measurement a photoplethysmogram (PPG) is obtained. It is an optically obtained plethysmogram, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption. With each cardiac cycle the heart pumps blood to the periphery. Even though this pressure pulse is somewhat damped by the time it reaches the skin, it is enough to distend the arteries and arterioles in the subcutaneous tissue. If the pulse oximeter is attached without compressing the skin, a pressure pulse can also be seen from the venous plexus, as a small secondary peak.
The change in volume caused by the pressure pulse may be detected for example by illuminating the skin with the light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. Each cardiac cycle appears as a downward peak in the photodiode. Because blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, and other circulatory conditions. Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the pulse oximeter is attached.
The implementation of the optical sensors as well as manufacturing thereof needs further improvement to obtain sensors which are more sensitive and accurate, and which are mechanically durable. If an optical sensor is provided as a module, it may have optical leakage wherein light leaks from the light source to the light detector already in the module causing problems in the measurement.
Especially the manufacture of wrist devices is challenging. Such devices are relatively small and usually contain the required electronics, display, optical components and other parts of the device all packed in a flat closed casing. Therefore the assembly of the parts requires extreme accuracy.